Hydrolysis of polyethylene terephthalate by ZSM-5 combined with supercritical carbon dioxide under neutral environment

Polyethylene terephthalate (PET) is a kind of stable polyester, which is difficult to be degraded in the natural environment, and the waste PET causes severe environmental problems. To achieve efficient and environmentally friendly hydrolysis of PET, we employed ZSM-5 as a catalyst for neutral hydrolysis of PET in the presence of supercritical carbon dioxide (SCCO2). This study demonstrates that the existence of SCCO2 in the system enhances PET hydrolysis efficiency. The swelling effect of SCCO2 on PET can relax the PET segment and promote the mass transfer of water molecules in the PET bulk phase. However, excessive pressure of SCCO2 leads to plasticization of PET which increases the crystallinity of PET and hinders hydrolysis. The activation energy was measured under different pressures, revealing a decrease in activation energy with increasing pressure below 7.5 MPa. When the pressure exceeds 7.5 MPa, the activation energy increases due to plasticization. Through the density functional theory (DFT) analysis combined with the experimental results, it has been confirmed that Brønsted acid on ZSM-5 plays a more important role than Lewis acid in the system.